One of the chief etiologic agents of dental caries is thought to be Streptococcus mutans, on the basis of epidemiologic studies in humans and evidence from laboratory experimentation which indicates that this organism and a diet high in sucrose constitutes severe cariogenic challenges to animals. The role of other oral microorganisms, including other streptococci which are prominent members of the microbial dental plaque in humans, is less certain. It is proposed to assess the cariogenic potential of human Non-Mutans plaque streptococci in hamsters under conditions which will be favorable to their implantation in the oral cavity. The methods proposed include, (a) the use of streptomycin resistant mutants of the test organisms to monitor the degree of implantation, (b) reduction of the sucrose level of the diet to sub- inhibitory concentrations and (c) incorporation of streptomycin in the diet to depress competing or antagonistic oral microbiota. The purpose of this study is to identify those organisms which may play a positive role in human caries causation as well as those which are less likely to do so. This information should be of value in planning human caries control procedures directed against the specific causative microorganisms.